Omnidirectional antennas, such as the common dipole and whip antennas, are the most widely used antennas. The omnidirectional antenna in the ideal case has a uniform radiation intensity about a center axis of the antenna, peaked in the plane perpendicular to the center axis. For example, the vertical dipole is an omnidirectional antenna with a uniform (constant) radiation intensity about its vertical axis (i.e., in the azimuth pattern) at any given elevation angle, and peaked at the horizontal plane.
In some modern practical applications, the class of omnidirectional antennas is broadened to include those with broad spatial coverage substantially symmetrical about a vertical axis over a span of elevation angles (mostly near the horizon in the context of terrestrial applications). However, some directionality or even nulls may be acceptable or even preferred in certain applications, especially in the digital wireless world. Nevertheless, the techniques in this disclosure provide for a substantially uniform azimuth pattern over a given span of elevation angles. In the elevation pattern, some beam tilt is generally unavoidable, and may be preferred in certain applications.
The proliferation of wireless applications is setting increasingly more demanding goals for wider bandwidth, lower profile, smaller size and weight, as well as lower cost for omnidirectional antennas. To achieve these physical and performance goals, the antenna engineer must overcome the Chu limit (Chu, L. J., “Physical Limitations of Omnidirectional Antennas,” J. Appl. Phys., Vol. 19, December 1948, which is incorporated herein by reference), which states that the gain bandwidth of an antenna is limited by the electrical size (namely, size in wavelength) of the antenna.
Specifically, under the Chu limit, if an antenna is to have good efficiency and fairly large bandwidth, at least one of its dimensions needs to be about λL/4 or larger, where λL denotes the wavelength at the lowest frequency of operation. At frequencies UHF and lower (below 1 GHz), the wavelength is longer than 30 cm, where the size of the antenna becomes an increasingly serious problem with decreasing frequencies (thus longer wavelengths). For example, to cover a high frequency band, say, 3-30 MHz, a broadband efficient antenna may have to be as huge as 15 m tall and 30 m in diameter.
To circumvent the Chu limit, one approach is to reduce the antenna height and trade it with larger dimensions parallel to the surface of the platform on which the antenna is mounted, resulting in a low-profile antenna. For example, when an antenna is mounted on a platform, such as the cell phone, or the earth ground, the platform becomes part of the antenna radiator, leading to a larger dimension for the antenna needed to satisfy the Chu limit. In many applications, low profile and wide bandwidth, such as “ultra-wideband,” have become common antenna requirements.
An “ultra-wideband” antenna is generally meant to have an octaval gain bandwidth greater than 2:1, that is, fH/fL≧2, where fH and fL are the highest and lowest frequencies of operation. Note that “ultra-wideband” is sometimes meant in practice to have two or more wide frequency bands (multi-band) with each band having an adequately wide bandwidth. A “low-profile” antenna is generally meant to have a height of λL/10 or less, where λL is the free-space wavelength at fL.
In the pursuit of wider bandwidth and lower profile, the traveling-wave (TW) antenna with its TW propagating along the surface of the platform was found to have not only an inherently lower profile but also potentially wider bandwidth. (The TW antenna is an antenna for which the fields and current that produce the antenna radiation pattern may be represented by one or more TWs, which are electromagnetic waves that propagate with a certain phase velocity, as discussed in the book “Traveling Wave Antennas” (Walter, C. H., Traveling Wave Antennas, McGraw-Hill, New York, N.Y., 1965, which is incorporated herein by reference), in which a number of low-profile TW antennas were discussed.)
Certain traveling-wave (TW) antennas, in which the TW travels either along or perpendicular to the surface of the platform, can have not only an inherently low profile but also potentially wide bandwidth. Further, the fields and current of certain TW antennas can produce an antenna radiation pattern that may be represented by one or more TWs.
FIG. 1 illustrates the progress of the omnidirectional TW (traveling wave) antenna toward broader bandwidth, miniaturization, and platform conformability in the prior art. The first stage, from (a) to (b), shows an early example of reduction in antenna profile. Here the high-profile whip antenna mounted on a platform is reduced to a low-profile transmission-line antenna (King, R. W. P., C. W. Harrison, Jr., and D. H. Denton, Jr. “Transmission-line missile antennas,” IEEE Transactions on Antennas and Propagation, vol. 8, No. 1, pp. 88-90. January 1960, which is incorporated herein by reference). Note that the whip antenna can be considered as a TW antenna, and specifically a 1-dimensional (1-D) normal-mode TW antenna. In effect, here the technique was to replace the high-profile normal-mode TW structure or source field with a low-profile 1-D transmission-line antenna, which is a 1-D surface-mode TW that provides a similar omnidirectional pattern coverage and vertical polarization like the vertical whip antenna.
While the 1-D surface-mode TW in the transmission-line antenna propagates in a path parallel to the ground plane (in other words, perpendicular to the z axis), its radiating current is mainly on one or more of its vertical posts parallel to the z axis with equivalent currents that are close to each other in phase from a relevant far-field perspective. Note that this 1-D surface-mode TW and its supporting structure do not have to be along a straight radial line about the z axis. For instance, the 1-D surface TW structure can be bent and curved in the x-y plane as long as the general characteristics of its 1-D transmission-line mode TW remain substantially intact and undisturbed.
However, the 1-D transmission-line antenna is inherently a narrow-band antenna. In general, only a few percent in bandwidth is achieved. Additionally, a lower antenna profile results in a smaller bandwidth. Several 2-D low-profile TW antennas exhibiting increasingly broader bandwidths, such as disk-loaded monopoles, blade antennas, etc. were then developed, as depicted in (b) to (c) of FIG. 1. Among them, the pillbox-shaped Goubau antenna (Goubau, G., “Multi-Element Monopole Antennas,” Proc. Army ECOM-ARO, Workshop on Electrically Small Antennas, Ft. Monmouth, N.J., pp. 63-67, May 1976, which is incorporated herein by reference) has a 2:1 bandwidth and a low profile of 0.065 λL in height (thickness), being nearest to the Chu limit. The spiral-mode microstrip (SMM) antennas, a class of 2-D TW antenna, represent a significant improvement in broadening the bandwidth and lowering the profile of the TW antennas, as shown in publications (Wang, J. J. H. and V. K. Tripp, “Design of Multioctave Spiral-Mode Microstrip Antennas,” IEEE Trans. Ant. Prop, March 1991; Wang, J. J. H., “The Spiral as a Traveling Wave Structure for Broadband Antenna Applications,” Electromagnetics, pp. 20-40, July-August 2000; Wang, J. J. H, D. J. Triplett, and C. J. Stevens, “Broadband/Multiband Conformal Circular Beam-Steering Array,” IEEE Trans. Antennas and Prop. Vol. 54, Nol. 11, pp. 3338-3346, November, 2006) and (U.S. Pat. Nos. 5,313,216, issued in 1994; 5,453,752, issued in 1995; 5,589,842, issued in 1996; 5,621,422, issued in 1997; 7,545,335 B1, issued in 2009), which are all incorporated herein by reference. The omnidirectional mode-0 SMM antenna has achieved practical octaval bandwidths of 10:1 or so and has an antenna height of about 0.09 λL and a diameter under λL/2. In the above examples, the Chu limit sets the lower bound of the operating frequency for an efficient antenna of a given electrical size, not its gain bandwidth.
A technique to reduce the size of a 2-D surface TW antenna is to reduce the phase velocity, thereby reducing the wavelength, of the propagating TW. This leads to a miniaturized slow-wave (SW) antenna (Wang and Tillery, U.S. Pat. No. 6,137,453 issued in 2000, which is incorporated herein by reference), which allows for a reduction in the antenna's diameter and height, with some sacrifice in performance.
The SW antenna is a sub-class of the TW antenna, in which the TW is a slow-wave with the resulting reduction of phase velocity characterized by a slow-wave factor (SWF). The SWF is defined as the ratio of the phase velocity Vs of the TW to the speed of light c, given by the relationshipSWF=c/Vs=λo/λs  (1)where c is the speed of light, λo is the wavelength in free space, and λs is the wavelength of the slow-wave, at the operating frequency fo. Note that the operating frequency fo remains the same both in free space and in the slow-wave antenna. The SWF indicates how much the TW antenna is reduced in a relevant linear dimension. For example, an SW antenna with an SWF of 2 means its linear dimension in the plane of SW propagation is reduced to ½ of that of a conventional TW antenna. Note that, for size reduction, it is much more effective to reduce the diameter, rather than the height, since the antenna size is proportional to the square of antenna diameter, but only linearly to the antenna height. Note also that in this disclosure, whenever TW is mentioned, the case of SW is generally included.
With the proliferation of wireless systems, antennas are required to have increasingly broader bandwidth, smaller size/weight/foot-print, and platform-conformability, especially for frequencies UHF and below (i.e., lower than 1 GHz). Additionally, for applications on platforms with limited space and carrying capacity, reductions in volume, weight, and the generally consequential fabrication cost considerably beyond the state of the art are highly desirable and even mandated in some applications.